As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased as an energy source for the mobile devices. Accordingly, much research on secondary batteries satisfying various needs has been carried out.
In terms of the shape of batteries, the demand for prismatic secondary batteries or pouch-shaped secondary batteries, which are thin enough to be applied to products, such as mobile phones, is very high. In terms of the material for batteries, the demand for lithium secondary batteries, such as lithium ion batteries and lithium ion polymer batteries, exhibiting high energy density, discharge voltage and power stability, is very high.
On the other hand, secondary batteries may be classified based on the construction of an electrode assembly having a cathode/separator/anode structure. For example, the electrode assembly may be configured to have a jelly-roll (winding) type structure in which long-sheet type cathodes and long-sheet type anodes are wound in a state in which separators are disposed respectively between the cathodes and the anodes or in a stack type structure in which pluralities of cathodes and anodes having a predetermined size are successively stacked in a state in which separators are disposed respectively between the cathodes and the anodes.
However, such conventional electrode assemblies have the following several problems.
First, the jelly-roll type electrode assembly is prepared by densely winding the long-sheet type cathodes and the long-sheet type anodes with the result that the jelly-roll type electrode assembly is circular or elliptical in section. Consequently, stress, caused by expansion and contraction of the electrodes during charge and discharge of a battery, may accumulate in the electrode assembly, and, when the stress accumulation exceeds a specific limit, the electrode assembly may be deformed. The deformation of the electrode assembly results in non-uniform gap between the electrodes. As a result, the performance of the battery may be abruptly deteriorated, and the safety of the battery may not be secured due to an internal short circuit of the battery. In addition, it is difficult to rapidly wind the long-sheet type cathodes and the long-sheet type anodes while uniformly maintaining the gap between the cathodes and anodes with the result that productivity is lowered.
Secondly, the stack type electrode assembly is prepared by sequentially stacking the plurality of unit cathodes and the plurality of unit anodes. As a result, it is additionally necessary to provide a process for transferring electrode plates, which are used to prepare the unit cathodes and the unit anodes. In addition, much time and effort are required to perform the sequential stacking process with the result that productivity is lowered.
In order to solve the above-mentioned problems, there has been developed a stack/folding type electrode assembly, which is a combination of the jelly-roll type electrode assembly and the stack type electrode assembly. The stack/folding type electrode assembly is configured to have a structure in which pluralities of cathodes and anodes having a predetermined size are stacked, in a state in which separators are disposed respectively between the cathodes and the anodes, so as to constitute a bi-cell or a full cell, and then a plurality of bi-cells or a plurality of full cells are wound in a state in which the bi-cells or the full cells are located on a long separator sheet. The details of the stack/folding type electrode assembly are disclosed in Korean Patent Application Publication No. 2001-0082058, No. 2001-0082059 and No. 2001-0082060, which have been filed in the name of the applicant of the present patent application.
FIGS. 1 and 2 typically illustrate an exemplary process for preparing a conventional stack/folding type electrode assembly.
Referring to these drawings, the stack/folding type electrode assembly is prepared, for example, by arranging bi-cells 10, 11, 12, 13 and 14 on a long separator sheet 20 and sequentially winding the bi-cells 10, 11, 12, 13 and 14 from one end 21 of the separator sheet 20.
A stack/folding type electrode assembly of FIG. 3, which is prepared using the above method, solves problems caused by the jelly-roll type electrode assembly and the stack type electrode assembly. When the stack/folding type electrode assembly is mounted in a battery case so as to prepare a secondary battery, however, the stack/folding type electrode assembly exhibits low safety. For example, when external impact is applied to the secondary battery, the electrode assembly may be pushed with the result that an internal short circuit may occur between cathode tabs 31 and a battery body or between anode tabs 32 and the battery body.
That is, when a certain object presses the battery due to external force, the cathode tabs 31 or the anode tabs 32 come into contact with an opposite electrode of the battery body with the result that a short circuit may occur. Electrode active materials react due to such a short circuit with the result that the temperature of the electrode active materials increases. Also, in a case in which a cathode active material is made of a lithium transition metal oxide exhibiting low electric conductivity, a large amount of heat is generated from the cathode active material due to such a short circuit with the result that combustion or explosion of the battery is further accelerated.
In addition, some of the anode tabs 32 may be cut during an anode V-forming process for welding the anode tabs 32 to an anode lead 33.
Consequently, there is a high necessity for an electrode assembly that can be prepared using a simple preparation process and secure a lifespan and safety of a secondary battery even when external force is applied to the secondary battery.